This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc, and, more particularly, to systems for centering a light beam on an information track on such a disc.
Video and other types of information can be stored on a disc as a succession of light reflective and non-reflective regions along a spiral information track on the surface of a disc-shaped record carrier. In reproducing the video information, a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type with which the invention may be employed, the intensity of the reflected beam takes the form of a high frequency carrier signal which is frequency modulated by the video information.
In order to store a reasonably large amount of information on the disc, successive turns of the information track must be so closely spaced that even a slight degree of disc eccentricity would cause the beam to traverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the information track.
In a commonly assigned copending application, Ser. No. 766,928, now U.S. Pat. No. 4,118,735, entitled "Synchronous Detection Tracking", of which Richard L. Wilkinson is the inventor, a dither tracking system is disclosed, wherein a constant-frequency dither signal is applied to oscillate the reading beam, in a radial sense, as it moves along the information track. The dither signal is multiplied by a filtered form of the reflectivity signal, to derive an error signal having a positive value when the central beam position is to one side of the track, and a negative value when the beam moves to the other side of the track. The error signal is added to the oscillatory dither signal, before application to a beam positioning transducer, which thereby maintains the dither motion substantially about the center of the information track. Although this technique has proved satisfactory for many applications, it requires the use of analog multiplier circuits, which necessitate critical circuit adjustments and are subject to drift in characteristics over a period of time. Accordingly, there has been a need for an improved and more reliable technique of dither tracking which avoids the use of analog multipliers. The present invention is directed principally to this end.